Floods

Heavy precipitation contributes to increased flooding, a pattern that has already been observed around the world.1 The frequency of great floods (100-year floods in large basins) increased substantially during the 20th century.2 However, flooding is also affected by various other human activities. For example, deforestation may exacerbate flooding, while changes in land use can greatly influence runoff, which can either increase or reduce the risk of flooding.

Runoff, or the surface water left over when the land cannot soak up any more, has also increased in many parts of the world, consistent with changes in precipitation.3 Regional shifts in precipitation can also increase the risk of flooding by raising water table levels, as seen in the northeastern United States.4

The warming climate is increasing the risks of both flood and drought, but at different times or in different places. For instance, the summer of 2002 in Europe brought widespread floods, but was followed a year later in 2003 by record-breaking heat waves and drought. In the summer of 2007, widespread flooding in central England (the wettest since records began in 1766) was accompanied by drought and record-breaking heat waves in southeast Europe.5

Flooding in large river basins, such as the Mississippi, is not driven by brief, extreme precipitation episodes alone. Rather, extreme precipitation must be sustained for weeks to months to flood the Mississippi. In spring, heavy rains on top of snow can contribute to flooding in northern regions. Such long-term, heavy precipitation episodes are becoming more common in some areas. In the U.S., 90-day periods of heavy rainfall were 20% more common from 1981 to 2006 than in any earlier 25-year period on record.6 Record breaking Mississippi flooding occurred in 2011 in association with very heavy rains, and was followed by extensive flooding further north in the Missouri River basin due to heavy rain and snowmelt.

Recent record floods, such as in Nashville, Tennessee in early 2010, in Pakistan in mid-2010, and in Australia in late 2010, were driven in part by the human-influenced trend toward heavy precipitation.7 Changes in large-scale patterns of atmospheric pressure also contributed to the Pakistan flooding.8